This invention relates to tandem mass spectrometry systems based on principles of analysis by time-of-flight (TOF). The object is to identify what molecules are present in a sample. The mass of a molecule may be readily measured by providing it with an amount of kinetic energy and measuring its velocity by time-of-flight techniques. However a number of different molecules may have the same mass and these can be distinguished from one another by dissociation and analysis of the masses of the daughters that are produced. In recent years there has arisen the need to analyse by tandem mass spectrometry, with the highest sensitivities, complex biological and other molecules and complex mixtures of molecules.